Regarding the lack of a certificate:
The value of a certificate can be debated and is relative in perspective of future employers.
From the students perspective it would serve as positive reinforcement. For example I held a few sales positions in telemarketing at $7.50 an hr. and the main motivator was 10-20% commission depending on sales. But I witnessed a striking difference between my first job to the second. At the second in addition to hourly & commission they would have contests. Most sales for day, week, and month, along with a playful and positive atmosphere. What they were doing worked so well that a year later I went back to my former employer trying to tell him about it and how he could triple the sales, but I was a young 22 yr old and of course he didn't want to take advice from a 22 yr old. I guess the point I'm trying to make is that something as simple as a fancy certificate given at the end of each course whether they chose to charge $1.99, $19.99, or be free, could and my logic tells me would increase users of Codecademy. Also you could look at from this perspective, Swag, like when Redbull gives out t-shirts. That person then wears the t-shirt and is a walking advertisement for Redbull.